The present invention relates to a transmission line having a short signal propagation delay time.
Heretofore, there has been proposed a transmission line as shown in FIG. 1. The transmission line 1 is made up of a signal conductor wire 2 placed at the center of the rectangular cross-section, a pair of conductors 3 placed on either side of the signal conductor 2, and an insulating resin covering jacket 4, such as polyethylene, which is called "form keeping resin material" having a rectangular cross-section. The conductor 2 and the conductors 3 are kept parallel to one another at the desired transverse separation distance. The conductors 3 act as the grounding wires for the signal conductor 2 and as mechanical reinforcement. Only one conductor 2 may suffice in some cases.
The transmission line 1 shown in FIG. 1 may be used alone or it may be used in a multiple component assembly. In the latter case, a plurality of transmission lines 1 are joined side-by-side by fusion bonding of the covering 4 so that they form a multiple flat cable 5 shown in FIG. 2. The distance between the signal conductors 2 is usually about 1.27 mm.
The conventional transmission line mentioned above has disadvantages. It has a relatively long signal propagation delay time because the electromagnetic wave resulting from signal transmission concentrates in the covering 4 made of polyethylene resin or the like. In the case of transmission line as shown in FIG. 1 employing polyethylene, the propagation delay time is about 4.7 nsec/m, and it has previously been impossible to reduce it below 4.0 nsec/m for the transmission line of this kind. For the characteristic impedance required, it is necessary that the conductors 3 be placed as far away as possible from signal conductor 2. Such an arrangement reduces the thickness of the covering 4 in the vicinity of the surface 4a. This can lead to insufficient dielectric strength when an electric current is applied to the conductor 3 while the transmission line is used under water, for example. Moreover, in the case of multiple component flat cable, it is necessary to keep adjacent conductors 2 away from one another.
The present device is intended to overcome the above-mentioned disadvantages inherent in the conventional transmission line of this kind, and to provide a transmission line having improved transmission characteristics. According to the device, there is provided a transmission line comprising a signal conductor, conductors placed in parallel relationship to said signal conductor, an insulating porous resin inner covering in which are enclosed said signal conductor and said conductors, and an outer covering.
Applicant's copending application U.S. Ser. No. 723,327, filed Apr. 15, 1985, discloses an electrical transmission line comprising at least one elongate signal conductor, one or more other conductors placed away from and substantially parallel to said signal conductor, all conductors encased in an outer insulating resin covering having a rectangular cross-section, the signal conductor(s) being further encased in an inner insulating porous resin covering, the porous resin covering having an electrical shielding layer thereover.
Applicant's copending application U.S. Ser. No. 723,448, filed Apr. 15, 1985, discloses a flexible cable for delivering power or force to, for example, a robot. That cable comprises at least one elongate power conductor, one or more linear reinforcing members spaced apart from and in substantially parallel relationship with the power conductor, the power conductor being encased within an inner, low-friction insulating covering, and all components further encased within an outer plastic covering having a substantially rectangular cross-section. Multiple component cables having individual components are also provided.